This invention is related to a transmission mechanism, particularly to that type having a new crank matched with a plastic-moulded gear for use on gymnastic bicycles.
Tranditionally, transmission mechanisms used in gymnastic bicylces are as that shown in FIG. 1. The structure of the known transmission mechanism is usuaslly formed by rigidly connecting a metal gear 1 with a crank 2 through a transmission mandrel 3 which is axially secured within a central axle position of the gear 1 and the crank 2. In this disposition an arch-shaped notch 3a formed at the head portion of the transmission mandrel 3 is parallely aligned with a through aperture 2a which is located across the horizontal axle position of the crank 2, and a dowel pin 4 with a longitudinally inclined portion is inserted into the through aperture 2a and closely engaged with the arch-shaped notch 3a so as to force the crank 2 and the mandrel 3 to be tightly connected to perform the transmission operation. As shown in FIG. 2, when the dowel pin 4 is inserted into the through aperture 2a, its inclined portion must be fully engaged with the mandrel 3 in order to force the mandrel 3 upward therefrom and press it tightly against the crank 2, which being raised, raises in turn the linked gear 1. As a result, an off-center condition exists in the transmission mechanism, and the assembly becomes unsteady with the gear 1 wabbling during rotation, rendering the gymnastic bicycle unstable. Therefore, not only is the revolving efficiency of the gymnastic bicycle reduced, but also the life expectancy of the transmission mechanism. Futhermore, the traditional metal gear is noisy, which is always bothersome to the rider and tends to lessen the interest of riding the bicycle.